Catadioptric optical system for scatterometry

Abstract

A catadioptric optical system having a high numerical aperture operates in a wide spectral range. The catadioptric optical system includes a correcting plate, a first reflective surface and a second reflective surface. The correcting plate conditions electromagnetic radiation to correct at least one aberration. The first reflective surface is positioned to reflect the electromagnetic radiation conditioned by the correcting plate. The second reflective surface is positioned to focus the electromagnetic radiation reflected by the first reflective surface onto a target portion of a substrate. The electromagnetic radiation reflected by the first reflective surface and focused by the second reflective surface is not refracted by a refractive element, thereby enabling the catadioptric optical system to operate in a broad spectral range.

1. A catadioptric optical system, comprising:
 a correcting plate configured to condition electromagnetic radiation to correct at least one aberration; 
 a first reflective surface positioned and configured to reflect a first portion of the conditioned electromagnetic radiation and to refract a second portion of the conditioned electromagnetic radiation; and 
 a second reflective surface positioned and configured to focus the reflected first portion of the conditioned electromagnetic radiation onto a target portion of a substrate, 
 wherein the first portion of the electromagnetic radiation is not refracted by an optical element, thereby allowing the catadioptric optical system to operate in a broad spectral range. 
 
   
   
     2. The catadioptric optical system of  claim 1 , wherein the first reflective surface comprises a convex reflective surface. 
   
   
     3. The catadioptric optical system of  claim 1 , wherein the second reflective surface is a surface of a monolithic glass element. 
   
   
     4. The catadioptric optical system of  claim 3 , wherein the first reflective surface is positioned on a second surface of the monolithic glass element. 
   
   
     5. The catadioptric optical system of  claim 1 , wherein the first reflective surface is positioned between the second reflective surface and the substrate. 
   
   
     6. The catadioptric optical system of  claim 5 , wherein the first reflective surface is positioned on mechanical supports. 
   
   
     7. The catadioptric optical system of  claim 5 , wherein the first reflective surface is positioned on a meniscus. 
   
   
     8. The catadioptric optical system of  claim 5 , wherein the second reflective surface is positioned between the correcting plate and the first reflective surface, and the second reflective surface comprises a hole, wherein the second reflective surface is positioned such that the conditioned electromagnetic radiation passes through the hole. 
   
   
     9. The catadioptric optical system of  claim 1 , wherein the second reflective surface comprises a concave aspheric reflective surface. 
   
   
     10. The catadioptric optical system of  claim 1 , wherein the catadioptric optical system is included in both a sensing branch and an alignment branch of a scatterometer. 
   
   
     11. An optical system for scatterometry, comprising:
 an optical element configured to deflect electromagnetic radiation; 
 a refractive element configured to condition the electromagnetic radiation to correct coma in the optical system; and 
 an objective system comprising a monolithic glass element optically contacted with the optical element and configured to receive the deflected electromagnetic radiation, the monolithic glass element comprising:
 a first surface configured to reflect the electromagnetic radiation, 
 a second surface configured to receive the electromagnetic radiation from the first surface and to reflect and focus the electromagnetic radiation toward a target portion of a substrate, and 
 a third surface, wherein the electromagnetic radiation reflected and focused by the second surface exits the monolithic glass element substantially perpendicular to the third surface, 
 
 wherein the optical element and monolithic glass element are comprised of the same type of material. 
 
   
   
     12. The optical system of  claim 11 , wherein the optical element comprises:
 a tilted surface configured to reflect the electromagnetic radiation, the tilted surface having a hole therein; and 
 an aspheric surface configured to reflect the electromagnetic radiation reflected by the tilted surface, thereby causing the electromagnetic radiation to pass through the hole in the tilted surface and impinge upon the first surface of the monolithic glass element. 
 
   
   
     13. The optical system of  claim 11 , wherein the first surface comprises a flat annulus and the third surface is positioned in the center of the annulus. 
   
   
     14. The optical system of  claim 11 , wherein the optical element and the monolithic glass element comprise fused silica (SiO 2 ). 
   
   
     15. A catadioptric optical system for scatterometry, comprising:
 a refractive lens group configured to condition electromagnetic radiation to correct one or more aberrations of the catadioptric optical system; 
 a first monolithic glass element configured to receive the electromagnetic radiation from the refractive lens group and is adapted to focus the electromagnetic radiation into a first focused spot; and 
 a second monolithic glass element configured to receive the electromagnetic radiation from the first monolithic glass element and is configured to focus the electromagnetic radiation into a second focused spot on a target portion of a substrate, 
 wherein the second monolithic glass element comprises,
 a flat reflective surface configured to reflect electromagnetic radiation entering the second monolithic glass element through a first spherical refractive surface, and 
 an aspherical reflective surface configured to reflect and focuses the electromagnetic from the flat reflective surface into the second focused spot; and 
 
 wherein the electromagnetic radiation enters the second monolithic glass element substantially perpendicular to the first spherical refractive surface and exits the second monolithic glass element substantially perpendicular to a second spherical refractive surface. 
 
   
   
     16. The catadioptric optical system of  claim 15 , wherein the first spherical refractive surface is concentric with the first focused spot, and the second spherical refractive surface is concentric with the second focused spot. 
   
   
     17. The catadioptric optical system of  claim 15 , wherein the first monolithic glass element comprises:
 a first reflective surface having a first central transparent portion; and 
 a second reflective surface having a second central transparent portion; 
 wherein the first and second reflective surfaces are positioned such that electromagnetic radiation enters the first monolithic glass element through the first central transparent portion and exits the first monolithic glass element through the second central transparent portion.